The transfer of personnel from a marine transport vessel such as a small boat to a large static construction or a large vessel is always problematic. The differential movement between the transfer and destination points creates a danger to personnel, particularly when access to the larger construction or vessel is in the form of a ladder.
The lighter the transport vessel/craft or the larger the waves, the more dangerous will be the transfer process. These problems are common to transfers from small transport vessels e.g. pilot boats to large tankers and containerships and to fixed constructions e.g. offshore turbines and gas and oil rigs.
There are a number of factors that define practical and safe transfers. Of these, the most obvious is wave size (amplitude) and speed. The higher and steeper the wave and the greater the frequency, the more dangerous the transfer. The smaller and the lighter the transport vessel, the greater the movement on the waves and the greater the volatility of the transport vessel movement.
At present, the wind turbine industry considers that a significant wave height of 1.5 meters represents the maximum safe conditions to allow the transfer of personnel onto offshore turbines. The fact that large Atlantic waves can routinely achieve a wave height of 6 meters shows how this factor limits access to offshore turbines. Oceanographic data shows that waves of up to 12 meters can be expected in Atlantic waters.
It is expected that turbines will be congregated in scores of structures. The offshore industry in the UK is so new and undeveloped that there is no certainty as to the type of marine transport vessel that will be required to provide access. There are the small fast transport vessels already providing access in the Thames Estuary, there are converted trawlers and there is the possibility of small fast inflatables launched from mother-vessels. The fact that offshore wind farms are generally expected to be built 20 to 40 km off the coast, and around 80 km off the coast for the UK round 3 wind farm sites, means that marine transport and access vessels will routinely experience harsh wave and tidal conditions. The industry is not developed enough to predict what will become the standard equipment and system of access for offshore waters.
What is certain is that the distance of the turbines offshore will require that transport vessels launched to access them must provide a high probability of achieving a successful delivery. The cost of aborting access programmes far offshore will be considerable. The types of transport vessels and the access mechanisms on the turbines will have to ensure a high success rate, even in the conditions to be anticipated in the North Sea and the near Atlantic.
Figures indicate that availability (i.e. the percentage of time that a turbine is ready to produce power if the wind is blowing) is lower for offshore turbines compared to onshore turbines. Onshore turbines have availability of approximately 98%, whereas many offshore turbines have availability of approximately 80-90%, due to maintenance time lost due to access restrictions. Lower availability levels are typically experienced in the winter months, when weather conditions are worse.
At present, most transport vessels delivering engineers to a turbine are driven hard into the large steel vertical tubes that are mounted on the turbine structure on either side of the access ladder. The captain maintains full throttle thus holding the bow of the transport vessel hard against the vertical tubes. With the bow pressed hard against the turbine and acting as the pivot for movement of the transport vessel, the larger the waves are the greater is the vertical movement of the stern. With a transport vessel 12 meters long, waves with a height of 4 meters would cause the transport vessel to adopt an inclination of 30 degrees to the horizontal. It is for this reason that transport vessels do not normally operate in a significant wave heights of greater than 1.5 meters.
With the transport vessel temporarily jammed in place the engineers can gain access to the ladder by stepping from the transport vessel to the ladder. At present, as the engineer stands on the bow of the transport vessel, he is able to pull down and clip himself onto the hook of an inertia reel device mounted on the access/work platform of the turbine. This device will catch him were he to fall. However, this device cannot differentiate between the downward plunge of the bow of the transport vessel with a fall by the engineer. If it detects a substantial downward movement it locks. This detaches the engineer from the deck of the descending transport vessel and into the air where he is vulnerable to a number of mishaps, including the possibility of being crushed between the transport vessel and the turbine. In addition, such equipment does not provide assistance in the climb up the ladder. Since engineers are dressed in heavy and cumbersome survival suits, this climb is laborious and potentially dangerous.
Some transport vessel operators in the industry contend that this “ramming technique” as currently practiced is damaging to both the transport vessels and the turbines. It is said that hulls and driving engines and gearboxes of transport vessels are damaged by the large pressures exerted upon them which they were never designed to resist. The same is true of the turbine structures themselves. As the industry moves into more difficult waters the attempt will be to achieve higher access percentages by the use of transport vessels with larger and larger horsepower. Ocean going tugs capable of developing 3000 HP are discussed as a solution. Applying such force to turbine structures is, it is thought probable, very likely to damage turbine foundations. As the industry moves into higher latitudes where wave heights and accelerations are far more substantial this policy of locking onto turbines using sheer force is likely to cause damage to transport vessels and turbine foundations.
Many solutions to the transfer problem have been proposed, from the technically extreme to the simple. The difficulty in achieving a completely satisfactory transfer method is evidenced by the fact that operators are considering the use of helicopters for the transfer of engineers despite a price tag of £5,000 per hour or more.
There is therefore a need for an improved method for the transfer of personnel between a marine transfer vessel such as a pilot boat and a larger construction such as a turbine, gas and oil rig, a larger offshore vessel, lighthouse or pier.